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Terminal Program Revisited
Port[ComPort[1] + FCR] := $07; {clearing the FIFOs}
Disabling the FIFOs look like this:
Port[ComPort[1] + FCR] := $00; {disabling FIFOs}
We will be using the FIFOs in the next section, so this is more a brief introduction to this
register so far.
4.7.3 Working with the PIC
Up until this point, we didn't have to worry about working with the Programmable Interrupt
Controller (the PIC). Now we need to. There isn't the need to do all of the potential
instructions for the PIC, but we do need to enable and disable the interrupts that are used
by the UART. There are two PICs typically on each PC, but due to the typical UART IRQ
vector, we really only have to deal with the master PIC.
Pic Function
PIC Commands
Interrupt Flags
I/O Port Address
0x20
0x21
This adds the following two constants into the software:
{PIC Constants}
MasterPIC = $20;
MasterOCW1 = $21;
After consulting the PIC IRQ table 6 we need to add the following line to the software in
order to enable IRQ4 (used for COM1 typically):
Port[MasterOCW1] := Port[MasterOCW1] and $EF;
When we do the "cleanup" when the program finishes, we also need to disable this IRQ as
well with this line of software:
Port[MasterOCW1] := Port[MasterOCW1] or $10;
Remember that COM2 is on another IRQ vector, so you will have to use different constants
for that IRQ. That will be demonstrated a little bit later. We are using a logical and/or
with the existing value in this PIC register because we don't want to change the values for
the other interrupt vectors that other software and drivers may be using on your PC.
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http://en.wikibooks.org/wiki/Serial%20Programming%3A8250%20UART%20Programming%23PIC%
20Device%20Masking
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